This invention relates to loudspeakers, and more particularly to novel means by which to couple the driven component of a loudspeaker to the atmosphere.
Loudspeakers and other types of sound reproducers heretofore have been coupled directly to the atmosphere using such devices as Helmholtz resonators, acoustic suspensions, infinite baffles, tuned ports and others to alter the out-of-phase sound emanating from the rear of the driving unit. These devices provide a "system resonance" intended to reinforce low frequency drivers so as to achieve a "flat" response curve. Such devices introduce sounds that are not present in the original music and they also cause distortions created by their sharply defined boundaries. These artificial sounds and distortions are further amplified when they are fed through exponential horns.
Exponential horns are recognized as very effective devices for coupling sound reproducers to the air which is to carry the sound to the human ear. The primary disadvantage of an exponential horn resides in the excessive length, from its inlet to its outlet, required to transport without distortion, those sound frequencies at the low end of the audible spectrum.
Historically, it has been recognized that an exponential horn designed to produce an undistorted audible note of 30 Hertz from a 15 inch diameter woofer, requires that the loudspeaker be confined in a chamber of no less than 5200 cubic inches and having an outlet of 75 square inches matching the inlet opening (throat) of the horn, and that the horn must exceed 16 feet in length and terminate in a mouth opening equivalent to 127 square feet, or about 11 feet square. Further, if the inlet end of the horn is provided with a larger cross sectional area, for example to at least match the effective cross sectional area of a larger loudspeaker diaphragm, the outlet end of the exponential horn is even more unreasonably large. In any event, the folding of such a path requires an unacceptably large cabinet, at least for residential usage.
In order to utilize at least some of the benefits of an exponential horn, it has been the practice heretofore to couple the diaphragm of a loudspeaker to the inlet of the horn by means of a "slot" formed by a chamber which communicates with the loudspeaker and which also has an outlet "slot" or passageway, of smaller dimensions than the loudspeaker diaphragm but matching the inlet end of the horn. The cross sectional area of the chamber changes at random from the diaphragm to the slot. The cross sectional area of this outlet passageway generally is greater than about one-fourth the cross sectional area of the loudspeaker diaphragm. Nevertheless, such a reduction in cross sectional area of such a "slot", relative to the loudspeaker diaphragm, allows the dimensions of the exponential horn to be reduced to a degree that allows the horn to be folded within a cabinet of overall dimensions which render it practicable for use at least in large rooms. On the other hand, the size of such a cabinet is unsuitable for use in the average home, and further size reduction, through further reduction in the dimensions of the coupling slot diminishes the quality of sound reproduction to an unusable level.